Luckily there is a pair of them close to you right at the beginning of the game.

Go straight east a couple streets from the first safehouse (Franklin’s House) to find the hospital.

The below pic was from GTA Online, so please disregard the mission markers.

On the northwest corner of the Hospital near the emergency vehicle entrance, look for a ladder to climb up. Go up this section, and you’ll find one more ladder that leads to the helipad, where you’ll find two helicopters waiting for you.